The invention relates to an improved direction finder for finding the azimuth of a target source of RF signals, and, more particularly, to such a direction finder including means for accurately determining the azimuth of a target source in a system having a relatively small signal to noise ratio.
Prior art direction finding systems have typically employed a circular antenna array to receive the signals that are transmitted by a particular RF target source. Thus, the RF signal that is received by each antenna is shifted in phase with respect to the corresponding signals of adjacent antennas in the array.
In operation, an electronic commutator is used to successively sample the RF signals of each of the antennas of the array and the sampled RF signals are typically converted to corresponding IF signals that are then applied to a tracking phase-locked loop (PLL). The output of the PLL corresponds to the magnitude of the phase shift between successive antennas of the array.
The PLL output signal is applied to a sample and hold circuit to provide a stair-case approximation of a sinusoidal waveform that represents the differential phase shift of the RF signal between adjacent antennas in the array. This stair-case signal is then passed to a band pass filter having a center frequency equal to the sampling frequency of the commutator and the zero crossing of the output of this filter identifies the antenna that is closest in line to the target source. The position of the zero crossing antenna with respect to a north reference antenna then provides an approximation of the azimuth of the target source.
In such prior art systems, large system noise and the 360.degree. ambiguity in the PLL tracking of a noisy phase angle will cause large and erroneous jumps in the incremental phase-shift measurements between adjacent antennas unless the antennas of the array are closely spaced. However, if the antennas of the array are closely spaced, the magnitude of the incremental phase shift between adjacent antennas will be decreased, thereby decreasing the signal to noise ratio of the received signals and reducing the accuracy of measurement of the signals. Thus, as system noise increases, the accuracy of such prior art systems is rapidly reduced.
Accordingly, it is an object of the invention to provide a direction finding apparatus for accurately determining the azimuth of a signal source in systems having relatively small signal to noise ratios.
A further object of the invention is to provide such an apparatus having a gradual rather than catastrophic degradation in accuracy with increasing system noise.
Another object of the invention is to provide a direction finding apparatus that makes optimal use of all of the phasor data that are received by the antennas of a direction finding array.
These and other objects of this invention will become apparent from a review of the detailed specification which follows and a consideration of the accompanying drawings.